As the density of semiconductor devices increases, the demands on interconnect layers for connecting the semiconductor devices to each other also increases. Therefore, there is a desire to switch from the traditional aluminum metal interconnects to copper interconnects. Unfortunately, suitable copper etches for a semiconductor fabrication environment are not readily available. To overcome the copper etch problem, damascene processes have been developed.
In a damascene process, the IMD is formed first. The IMD is then patterned and etched. The barrier layer 14 and a copper seed layer are then deposited over the structure. The barrier layer 14 is typically tantalum, tantalum nitride or some other binary transition metal nitride. The copper layer is deposited by electroplating over the seed layer. The copper is then chemically-mechanically polished (CMP′d) to remove the copper from over the IMD 16, leaving copper interconnect lines 18 as shown in FIG. 1. A metal etch is thereby avoided.
Barrier layer 14 is required because copper has high diffusivity. Copper interconnects totally rely on the encapsulating barrier materials to prevent copper from diffusing through to cause leakage and transistor poisoning. The basic requirements for the barrier materials are 1) good barrier efficiency, 2) good copper wettability, and 3) strong copper to barrier bonding. The most commonly used barrier materials to date are TiN, Ta, and TaN. TiN and Ta do not meet the first criterion very well although Ta is a little better than TiN. If TiN or Ta is used as the barrier material in copper interconnects, a relatively thick layer of barrier material is required in vias and trenches. This becomes less and less viable as device geometries shrink. TaN has near amorphous structure and therefore, it has a better barrier efficiency. However, TaN, does not meet the last two criteria well. Copper does not like to wet to TaN surfaces and tends to detach from TaN due to the weak bonding strength. This causes voids in the copper during the via fill and negatives impacts the electromigration performance. Metal-silicon-nitrides have better wetting properties. Unfortunately, current methods of forming these metal-silicon-nitrides are difficult to perform and result in a film having high resistivity.